Containers serve to hold contents, for example to hold liquids or powdered foodstuffs or other substances. The containers have an opening or mouth. It is often desirable or even necessary to close this container mouth with a disc-shaped seal in order to tightly seal off the contents from external influences.
There are a number of reasons for the requirement of this tight seal. On the one hand, the contents need to be protected against interfering influences from the outside, such as, for example the penetration of water vapor or oxygen; on the other hand, the contents also need to remain odor-tight. There is another reason in the case of aggressive filling materials, for which a leakage protection that is as optimal as possible needs to be provided. Finally, such a tight seal can also afford a tamper-evident closure for commerce, because a user can recognize immediately whether someone has already manipulated the container contents beforehand.
In addition, the container opening is then closed additionally with a screw cap or a similar element, which provides for the mechanical and stable closure of the container opening outside of the disc-shaped seal. In this case, the disc-shaped seal itself is a flexible thin film. On first-time use after acquisition, the user unscrews the screw cap and breaks the film in order to obtain the contents of the container. Afterward, he reseals the container if the entire contents have not yet been withdrawn. This renewed closure occurs by screwing the screw cap back onto a corresponding counterthread on the container opening, now without the film, which has meanwhile been broken and removed. The screw closure can close the opened contents, if need be, possibly for a period of time that is not as long as the tamper-evident closure configuration, but is nonetheless a satisfactory period of time for the user, who can control it himself from then on.
The film that seals the container contents, such as is known from EP 0 717 710 B1, for example, is applied by means of induction sealing. To this end, a complete sealing disc is put in place, the bottommost layer of which forms the sealing layer. Lying above the latter is a metallic second layer, generally consisting of aluminum, which serves for thermal coupling and heat transfer during the induction process and, if need be, forms an additional mechanical protection. The second layer is bonded to the first layer tightly and especially in a manner that provides good conduction for heat transfer. If need be, yet further layer-like components of the sealing disc, which remain in the cap when the screw closure or other rotating closure is opened, are then provided above the mentioned aluminum layer.
A constant problem in the case of such closures, which are intrinsically well proven and reliable, is the initial opening by the user or consumer. Breaking of this film or group composed of the lower layers, which are situated on the mouth of the container when it is first opened, by a knife or by the finger of the user is not possible or hygienic for all container contents, on the one hand, and leaves the rest of the film on the edge of the opening of the container, on the other hand, which can impede the container contents from later being poured or shaken out.
In order to make opening more convenient for the user and also safer for further use, the user is provided with a grip for pulling off the film. For example, this grip can be a grip tab that projects outward, as proposed in DE 39 20 324 A1 and EP 0 408 217 A1, which thus can then be gripped by the user outside the edge of the mouth and pulled upward, so that the entire film detaches from the container edge.
However, this quite simple and intrinsically satisfying concept has the drawback that the grip tabs interfere with the screw cap when it is screwed in place, because they need to be accommodated in the thread and screw cap in a suitable form. On the one hand, little space is available there and, on the other hand, these grip tabs can also impede the function of the screwing operation. Conversely, the grip tabs can also be damaged or, in the worst case scenario, even the entire sealing disc impeded in its sealing function by the screwing operation.
In proposals taken from EP 0 668 221 B1, GB 2 330 134 A, JP 2000-191021 A, and DE 10 2007 014 084 B3, the sealing discs are furnished with grip tabs for which the thickness of the grip tab is less than the thickness of the other surface areas of the sealing disc so as to reduce the problem encountered with grip tabs that project into the threads of screw caps. This is accomplished by having only one or a few of the plurality of layers of the sealing disc also form the grip tab.
Owing to the fact that the tabs of the sealing discs are designed to be very thin, they can be accommodated far more flexibly in the remaining empty spaces within the screw cap and the screw cap thread. The risk of mutual destruction of the individual elements when the packaging is closed or even during the opening operation is thereby appreciably reduced.
Alternatively to or also in combination with this design of the grip tab with a lesser thickness, it is known from EP 2 045 194 B1 and WO 2010/115811 A1 to have the grip tabs not project outward into the thread, but rather to have them double over and insert back onto or between specific layers of the sealing disc.
However, this has other drawbacks. As has been found, such a doubling over into the surface area of the mouth onto the other layers of the sealing disc interferes with the induction sealing properties of the entire sealing disc composite. Namely, a double aluminum layer is present in this way in a subarea of the otherwise circularly symmetrical arrangement and this appreciably alters the induction sealing properties.
The same drawbacks arise in a second embodiment taken from EP 2 045 194 B1, published later. This embodiment doubles over on the grip tab, initially with all layers folded back by 180° on the top side of the sealing disc and then folded back on themselves a second time by 180°. This has the further drawback that the aluminum layer is overlaid three times in the area of the grip tab, thereby leading to a striking asymmetry of the induction properties.
As a result of an asymmetry of the induction properties, the area of the grip tab is not sealed tightly and cleanly enough, when a container is sealed, as is the sealing disc on the rest of the circumference of the opening of the container. Of course, attempts will be made to circumvent this problem by way of appropriate arrangements of the induction devices or skillful control. However, on the one hand, this is very tedious and, on the other hand, the basic problem of an asymmetric sealing remains. In particular, there remains the risk that, although the majority of all containers that are to be sealed are properly sealed owing to these additional measures, there is a higher likelihood of an outlier or a deficient sealing in isolated cases.
As a result, the user is then confronted with a container having a product that is already spoiled because it has not been properly stored in a sealed container or else he has the feeling that at least something is not right about the product or that someone has possibly already pulled on the tab before him in an attempt to open the product.
This behavior when the container is torn open by means of a tab is felt to be unpleasant and unreliable and hence rejected by the user. Here, it needs to be considered that a user is not aggravated just in this isolated instance, but will also extend his general rejection to the respective product series of the filler of the container and possibly will dispense with purchasing such goods in future instances, so that the manufacturer of the contents of containers that are to be packaged can also suffer damage to its image.
Therefore, the problem of the present invention is to propose a sealing disc by means of which the problems mentioned can be prevented to the greatest extent possible.